Split_Part w/negative integer does not work

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Page: https://www.postgresql.org/docs/15/functions-string.html
Description:

When n is negative, an error occurs.

Documentation
split_part ( string text, delimiter text, n integer ) → text
Splits string at occurrences of delimiter and returns the n'th field
(counting from one), or when n is negative, returns the |n|'th-from-last
field.
split_part('abc~(at)~def~@~ghi', '~(at)~', 2) → def
split_part('abc,def,ghi,jkl', ',', -2) → ghi

Observation
SELECT split_part('abc,def,ghi,jkl', ',', -1);
generates
ERROR: field position must be greater than zero
SQL state: 22023

Or in code
DO $$
DECLARE
mytxt text;
BEGIN
mytxt = split_part('abc,def,ghi,jkl', ',', -1);
RAISE NOTICE '%', mytxt;
END $$

ERROR: field position must be greater than zero
CONTEXT: SQL statement "SELECT split_part('abc,def,ghi,jkl', ',', -1)"
PL/pgSQL function inline_code_block line 6 at assignment
SQL state: 22023